The present disclosure relates to directional or steerable beam antennas, of the type employed in such applications as radar and communications. More specifically, it relates to leaky-waveguide antennas, of the type including a dielectric feed line (i.e., a potentially leaky waveguide) loaded with scatterers (antenna elements), where coupling between the scatterers and the feed line can be altered by switches, whereby the antenna's beam shape and direction are determined by the pattern of the switches that are respectively turned on and off.
Steerable antennas, particularly leaky-wave antennas, are capable of sending electromagnetic signals in, and receiving electromagnetic signals from, desired directions. Such antennas are used, for example, in various types of radar, such as surveillance radar and collision avoidance radar. In such antennas, the receiving or transmitting beam is generated by a set of scatterers coupled to the feed line or waveguide. Interacting with the feed line, the scatterers create leaky waves propagating outside of the feed line. If the scatterers are properly phased, they create a coherent beam propagating in a specific direction. The leakage strength and phase caused by each scatterer depend on the geometry and location of the scatterer relative to the feed line or waveguide. The coupling strength can be controlled by changing the geometry of the scattering elements. Correspondingly, the shape and direction of the scattered beam can be controlled by varying the scatterer geometry or topology. The geometry (topology) of the scatterers can be electronically altered by using microwave (or other suitable) switches connecting parts of the scatterers. Thus, the shape and direction of the antenna beam can be controlled electronically by changing the state of the switches. Different ON/OFF switch patterns result in different beam shapes and/or directions.
Any of several types of switches integrated into the structure of the antenna elements or scatterers may be used for this purpose, such as semiconductor switches (e.g., PIN diodes, bipolar and MOSFET transistors, varactors, photo-diodes and photo-transistors, semiconductor-plasma switches, phase-change switches), MEMS switches, piezoelectric switches, ferro-electric switches, gas-plasma switches, electromagnetic relays, thermal switches, etc. For example, semiconductor plasma switches have been used in antennas described in U.S. Pat. No. 7,151,499, the disclosure of which is incorporated herein by reference in its entirety. A specific example of an antenna in which the geometry of the scattering elements is controllably varied by semiconductor plasma switches is disclosed and claimed in U.S. Pat. No. 7,777,286, the disclosure of which is incorporated herein in its entirety. Another example of a currently-available electronically-controlled steerable beam antenna using switchable antenna elements (scatterers) is disclosed in U.S. Pat. No. 7,995,000, the disclosure of which is incorporated herein its entirety.
FIG. 1 schematically illustrates a conventional steerable-beam antenna 10 comprising a single array 12 of switchable scatterers 14 coupled to a feed line or waveguide 16 extending along an axis x. Each of the scatterers 14 is switchable between an open state or state of low scattering L, and a closed state or state of high scattering H. Typically, in operation, the scatterers 14 will be selectively switched to low and high states to create a diffraction grating with P scatterers in each repetitive period Pd, where P includes N low-state scatterers and M high-state scatterers, and where d is the spacing between adjacent scatterers 14. In the illustrated example, the period P=5, comprising four L-scatterers and one H-scatterer. The resultant beam angle α will thereby be given by the equation (1):sin α=β/k−λ/Pd  (1)                where β is the wave propagation constant in the feed line 16, k is the propagation wave vector in a vacuum, and λ is the wavelength in vacuum. It will thus be seen that, by selectively switching the scatterers 14 between a high state and a low state, the grating period Pd can be controllably varied, thereby controllably changing the beam angle α of the electromagnetic radiation emanating from the feed line 16.        
The above-described antenna 10 may be viewed as a single array 12 of switchable scatterers 14 and a feed line 16 that feeds an electromagnetic signal to, or receives an electromagnetic signal from, the array 12. Each of the scatterers 14 is switchable between a low state L and a high state H. A specific pattern of H-state and L-state scatterers 14 represents a hologram that forms a coherent “leakage” (coupling between the free space and the feed line 16). By changing the pattern of H-states and L-states by means (for example) of a control signal source (not shown), the beam can be steered or manipulated in different ways, such as beam-steering, tracking, control of side lobes, multi-beam creation, control of beam width, etc.
In theory, in an ideal antenna, the L-state scatterers would not scatter electromagnetic power at all. In practice, however, real L-state scatterers still scatter a small amount of power. This so-called “parasitic” scattering degrades the desired steerable antenna beam, and may result in compromised radar resolution, detection of non-existing targets, etc. A beam pattern affected by parasitic scattering is illustrated graphically in FIG. 2, which charts relative antenna gain versus angle in a single array antenna. The steerable beam is labeled “A,” and the accompanied parasitic edge scattering is labeled “B.” The power level of the parasitic scattering is lower than that of the steerable beam A (about −20 dB relative to the peak gain of the steerable beam A in the illustrated example), but it still may degrade the antenna operation.
It would therefore be desirable to provide a mechanism for reducing the parasitic scattering in an electronically-controlled steerable beam antenna without measurably reducing the amplitude of the steerable beam.